WordPress doesn’t have a built in subscriber email list, but lots of plugins have that. What plugin or service are you using for subscriptions?
I’m using Feeburner Email Widget.
FeedBurner is a third party service. I do think you can to log in on their site to manage subscribers, including removing someone, but you will need to check with FeedBurner directly to make sure.
I searched Google and found a post that looks relevant, but I’m not sure whether it’s accurate. You can try it out if you’d like:
http://www.bloggerhowtoseotips.com/2013/12/How-to-Remove-Any-Subscribers-from-Your-Feedburner-Subscribers-List.html
If you find that solution works, you should reply back here to confirm your steps so it will help others who might have the same question in the future.